1. Field of the Invention
The present invention relates to an image forming apparatus, e.g., a copying machine or printer, which forms an image electrophotographically and, more particularly, to an image forming apparatus in which a toner-image-bearing transfer medium (to be referred to as a sheet hereinafter) after the fixing process can be correctly guided to a curl correcting means.
2. Description of the Related Art
According to some copying machine, a latent image formed on a photosensitive body through exposure (writing) in accordance with a document or image data is made visible by developing. The obtained toner image is transferred to the first surface (obverse surface) of a sheet by the operation of a transfer means, and is fixed by a heat roller type fixing unit. After that, the sheet is reversed by a reversing mechanism, and is fed to a transfer region again. Then, another toner image is transferred on the second surface (reverse surface) of the sheet. In this copying machine, conventionally, a curl (curling tendency) tends to occur in the sheet during the fixing process. Hence, after curling occurs, the sheet conveying performance degrades, and image transfer to the second surface of the sheet cannot be performed well.
In order to cope with these problems, a curl correcting means (curl correcting mechanism) may be added with which the sheet is drawn through a sponge roller pair by applying a force opposite to the direction of curl to the sheet after the fixing process, so that the curl is corrected. With this sponge roller method, the linear velocity of the sheet is difficult to manage. Simultaneously, the sheet cannot be regulated such that the leading end of the curled sheet is clamped well by the sponge roller pair. Thus, a satisfactory result cannot be obtained.
More specifically, for example, a sponge roller pair is provided at a position downstream of a fixing processing means and comparatively close to the fixing processing means. The fixing processing means has, as main constituent elements, an upper fixing roller incorporating a heat source, and a lower fixing roller which rotates in tight contact with the upper fixing roller. A guide member is provided at a position immediately before the sponge roller pair.
The guide member is arranged to guide the leading end of the sheet toward the nip position of the sponge roller pair.
It is, however, difficult to obtain an approach angle of the sheet for the nip of the sponge roller pair stably, and paper jamming and corner bent of the sheet occur often.
These inconveniences are apparently related to the arrangement itself of the curl correcting means and partly to the approach path of the sheet which extends from the fixing section to the curl correcting means.
The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus having such an arrangement that a sheet after the fixing process can be sent to a curl correcting means in a stable state.
In order to achieve the above object, according to the first aspect of the present invention, there is provided an image forming apparatus having an image carrier, image forming means for forming a toner image on the image carrier, transfer means for transferring the toner image formed on the image carrier onto a transfer medium, and fixing processing means including a pair of upper and lower fixing rollers for fixing the toner image onto the transfer medium, wherein the apparatus comprises curl correcting means, including a rotatable sponge roller and a fixed press member which presses the sponge roller while biting into part of an outer surface of the sponge roller, and provided downstream of the fixing processing means, and regulating means, provided immediately before the curl correcting means, so as to guide a leading end of the fixing-processed transfer medium to that position on the outer surface of the sponge roller which is tilted, within a range of 5xc2x0 to 10xc2x0 (both inclusive) from a reference line that connects a center of the sponge roller and a press start position of the press member with respect to the sponge roller, to a direction opposite to a rotating direction of the sponge roller.
According to the second aspect of the present invention, there is provided an image forming apparatus wherein the sponge roller according to the first aspect is rotated at a linear velocity larger than those of the pair of upper and lower fixing rollers, and the linear velocity of the sponge roller is 325 mm/s to 329 mm/s.
According to the third aspect of the present invention, there is provided an image forming apparatus wherein the sponge roller according to the second aspect has a rubber hardness of 2 degrees to 12 degrees (both inclusive) measured by Asker C measurement, and the press member bites into the sponge roller by a bite amount of 2.0 mm at a deepest portion.
According to the fourth aspect of the present invention, there is provided an image forming apparatus wherein the regulating means according to the first aspect comprises a first regulating member which has a regulating surface formed of an arcuate recessed surface formed on an approaching side of the fixing-processed transfer medium and a flat surface formed on an outlet side of the transfer medium continuously to the arcuate recessed surface, and a second regulating member which has a regulating surface formed of an arcuate projecting surface formed on an approaching side of the fixing-processed transfer medium and a flat surface formed on an outlet side of the transfer medium continuously to the arcuate projecting surface, and which is arranged to oppose the first regulating member.
According to the third aspect of the present invention, there is provided an image forming apparatus further having, downstream of the curl correcting means, a reversal delivery/re-feeding mechanism for reversely delivering and re-feeding the transfer medium which is curl-corrected through the regulating means and the curl correcting means according to the first aspect, wherein the regulating means further comprises a convey path switching member which can direct the transfer medium toward the reversal delivery/re-feeding mechanism.
As is apparent from the above aspects, according to the present invention, the arrangement of the regulating member which is located at a position where it is easily thermally affected by heat associated to fixing is improved, and sheet feeding to the curl correcting means can be performed stably. Thus, the curling tendency of the sheet is corrected reliably, and accordingly image formation on the second surface of the sheet can be performed well.